1. Field of the Invention
This invention relates to semiconductor processing and, more particularly, to a method for oxidizing one or more layers within a semiconductor topography.
2. Description of the Related Art
The following descriptions and examples are given as background information only.
Oxidation is a common technique used in semiconductor fabrication processing for forming a dielectric layer. More specifically, the diffusion of oxygen into a material is often used to form a dielectric within a semiconductor topography. For example, oxygen molecules may be diffused into silicon to produce silicon dioxide. In other cases, oxygen may be diffused into silicon nitride to produce silicon oxynitride. In yet other embodiments, oxygen may be diffused into metal such that a metal oxide may be formed. In any case, dielectric layers play a vital role within semiconductor devices. In particular, dielectric layers often serve to isolate metal layers. In some cases, dielectrics may additionally serve as a barrier layer to the diffusion of impurities from underlying or overlying layers. In yet other embodiments, a dielectric layer may serve as a tunneling layer for electrons. For example, a dielectric may be used within magnetic tunnel junctions (MTJ) of magnetic random access memory (MRAM) circuits such that tunnel magnetoresistance may be used to measure a differential resistance between magnetic states of the junction.
Consequently, in some cases, the compositional and/or dimensional characteristics of dielectric layers may directly affect the operation of devices formed from such layers. For example, the thickness of a dielectric may affect the insulating properties of the layer since the insulating properties of a given dielectric material generally increase with the thickness of a layer. In the case of a MTJ, the tunneling resistance of a junction is generally exponentially dependent on the thickness of the dielectric tunneling layer. As such, it is typically advantageous to fabricate the dielectric tunneling layer of a MTJ at a particular thickness such that an optimum tunneling magnetoresistance ratio may be obtained within the junction. In addition, the distribution of oxygen molecules within a layer may affect the insulating and tunneling properties of a dielectric layer. More specifically, non-uniformity of the placement of oxygen molecules along the lateral and vertical dimensions of the layer may cause the properties of the dielectric layer to be inhomogeneous.
Unfortunately, however, not all oxidation processes are able to produce dielectric layers with uniform distributions of oxygen. In addition, some oxidation processes are limited in the thickness of dielectric that may be generated from their processes. For example, the process commonly referred to as “natural oxidation” typically cannot oxidize more than a few angstroms of a particular material. For instance, natural oxidation typically cannot oxidize more than approximately 6 angstroms of aluminum metal. Similar thickness limitations generally exist for other materials oxidized by natural oxidation as well. In general, the process of natural oxidation causes the surface of a material to quickly passivate. Such surface passivation prevents oxygen molecules from diffusing further into the material, restricting the thickness of the resulting dielectric.
As such, in some cases, the process of natural oxidation may not produce a dielectric which provides adequate insulation, barrier, or tunneling properties for the device being fabricated. For example, in some embodiments, the thickness of an aluminum oxide tunneling layer of a MTJ may need to be approximately 15 angstroms in order to obtain adequate tunneling resistance through the junction. The aluminum layer used to fabricate a tunneling layer of such thickness may need to be approximately 10 angstroms thick prior to oxidation. Consequently, natural oxidation may not provide an aluminum oxide layer of sufficient thickness in such a case. In some embodiments, the natural oxidation process may be duplicated in order to oxidize an accumulative amount of aluminum that is approximately 10 angstroms thick. Such a process may be difficult to fabricate, however, due to the difficulty of depositing metal layers with uniform thicknesses of less than approximately 10 angstroms. In addition, such a process is undesirably long, increasing the processing time and, therefore, the costs of the fabrication process. In any case, a MTJ having an aluminum oxide layer produced from natural oxidation generally has a relatively low junction resistance. Consequently, the differential output voltage signal from such a junction may be undesirably low, reducing the probability of reading correct information from the device.
In addition, natural oxidation is often conducted at temperatures greater than approximately 300° C. in order to oxidize a layer with a targeted thickness and/or within a targeted time frame. For example, in some cases, the oxidation processes may be conducted between approximately 400° C. and approximately 1100° C. In some embodiments, exposure to such temperatures may undesirably affect the operation of a device. For example, a device having magnetic layers may be affected by a processing temperature greater than approximately 400° C. or, more specifically, greater than approximately 300° C., in some cases. In particular, a relatively high temperature may alter the magnetic direction or the strength of magnetization within magnetic layers of a device. In some embodiments, such a high processing temperature may cause individual magnetic layers within a structure to interdiffuse, causing the properties of the magnetic layers to change.
A method often used to oxidize relatively thick layers of metal in a timely manner and at a relatively low temperature is a process referred to as “plasma oxidation.” In general, plasma oxidation may refer to a process which generates a plasma comprising oxygen and directs ions from the plasma toward the surface of the layer being oxidized. The process is conducted within a single chamber reaction vessel and, therefore, is sometimes referred to as “direct plasma oxidation.” The diffusion of ions within a layer, however, may undesirably rearrange the atomic structure of the layer being oxidized. In particular, the diffusion of ions may cause the electrical and structural properties of a layer to change beyond what may be obtained through natural oxidation of such a layer. In addition, such a process may produce a dielectric with a non-uniform distribution of oxygen within the layer, causing inhomogeneous insulation, barrier, and/or tunneling properties. Furthermore, oxygen in such a process may, in some embodiments, diffuse to underlying layers before fully oxidizing the surface layer. Such a characteristic may be particularly undesirable for the fabrication of a tunneling layer of a MTJ. In particular, oxidizing a magnetic layer underlying a tunneling layer of a MTJ may reduce the magnetoresistance of the MTJ during operation of the MRAM circuit.
Therefore, it may be advantageous to develop a method which can efficiently oxidize a relatively thick layer of material without causing the problems described above. In particular, it may be beneficial to develop a method which can oxidize a layer at a temperature less than approximately 300° C. In addition, it may be advantageous to develop a method which can oxidize a portion of a layer having a thickness greater than approximately 6 angstroms without oxidizing underlying layers. Such a method may also be capable of oxidizing a portion of a layer having a thickness less than approximately 6 angstroms as well. In either case, it would be beneficial for such a method to produce an oxidized topography having a uniform distribution of oxygen such that a dielectric layer having homogeneous properties may be fabricated. Furthermore, it would be advantageous for such an oxidation method to diffuse oxygen into a layer without undesirably rearranging the layer's atomic structure beyond what may be obtained through natural oxidation of such a layer.